Art and antiques burglary is Britain's biggest theft at £80m

Police initially believed that the more than 300 art works and antiques stolen from the 17th-century Ramsbury Manor in Wiltshire in February were worth about £30m. But an audit has now placed the value at £80m, surpassing the £53m in cash taken from a Securitas depot in Kent three weeks after the burglary.

In the days following the raid on his home, Mr Hyams, who made an estimated fortune of more than £320m from building modernist tower blocks including London's Centre Point, told detectives he was not concerned at the financial loss.

But the new value of the missing items underlines the rarity of what was taken from Mr Hyams' extensive collection, described by police as being of "museum quality". Among the items taken in the burglary on 1 February was a £500,000 clock made by the 17th- century clockmaker Thomas Tompion, and a barometer made by his apprentice, Daniel Delander, which was bought by Mr Hyams in 1992 for £275,000.

Dozens of smaller items were taken in the raid, including paintings, silver, sculpture and porcelain. About 140 items were recovered last month from a storm drain on waste ground at Stratford-upon-Avon. The figure of £80m, reported by The Art Newspaper, remains an estimate, meaning the value could still rise further.

The burglary was carried out by thieves who cut through steeling cabling on the heavily-guarded perimeter fence at Ramsbury, near Marlborough, before ramming open a pair of wooden doors at the rear of the house, bought by Mr Hyams in 1964 for £650,000.

The 78-year-old developer, who fiercely guards his privacy, has quietly amassed what is reputed to be one of the finest private art collections in Britain. The thieves escaped with what friends described as a "small fraction" of his art works. Among the items left untouched were paintings by Rembrandt, Goya, Rubens, Turner and Gainsborough.

Mr Hyams has said that he intends to bequeath Ramsbury Manor and its contents to the nation.